The complexity and relationships of various classes of RNA in the mouse brain are being studied by nucleic acid hybridization strategies. A population of nonadenylated messenger RNAs (poly(A)-) equal in complexity to that of polyadenylated (poly(A) plus) mRNA has been discovered. In terms of MRNAs encoded by single copy DNA, there is essentially no sequence overlap between the poly(A) plus and poly(A)-mRNAs. The biological significance of these two classes of mRNA is unknown, but our recent investigations show that a wide variety, perhaps all, of the poly(A) plus mRNAs are encoded discontinuations in the genome (split genes). Other studies indicate that most poly(A) plus mRNAs are derived from much larger percursor nuclear RNAs and that these mRNAs are also regulated, in terms of their presence in the polysomes, post-transcriptionally. Preliminary experiments suggest that these observation of poly(A) plus mRNA may not apply to poly(A)-mRNA. Many brain-specific mRNAs may be nonadenylated.